This invention relates, in general, to a bonding method and apparatus and, more particularly, to a bonding method and apparatus for use in bonding leads to a semiconductor die.
Various types of bonding apparatus are known in the art. In general, these apparatus have a bottom thermode forming a base on which the die and lead frame rest. This bottom thermode is generally heated to a nominal temperature between approximately 150.degree. C. and 250.degree. C. A top thermode is then heated above the melting point of the materials to be bonded. For bonding a tin plated copper lead to a gold contact, the upper thermode is heated to 475-550.degree. C. The upper thermode is then lowered causing the leads to rest on the contacts. A temperature of 300-350.degree. C. is required for the bond to form between the tin plated copper lead and the gold bump. However, because of the efficiency of the die and lead frame to conduct heat, the thermode must be heated to 475-550.degree. C. The heat transfer from the gold bump to the substrate has been shown to result in microcracking of the silicon. This can cause problems in subsequent processing of the devices where the bump may be pulled out of the silicon.
Another problem in the prior art is that the upper thermode blade has a planar edge. Because of variances in die thickness across a single die, or between the thermode and the heater block, the gold bumps may be in a plane different from that of the upper thermode blade. This results in either exerting too much pressure on the higher bumps or not bonding the leads to the lower bumps.
Accordingly, it is an object of the present invention to provide a bonding method and apparatus which overcomes the above deficiencies.
Another object of the present invention is to form a bond between the lead frame and the die which will not damage the substrate.
Still another object of the present invention is to provide a bonding apparatus which is compliant and will adapt to variances in bump planarity.
The above and other objects and advantages of the present invention are provided by the bonding method and apparatus described herein.